There have been demonstrated a large number of ways to generate light for use in displays. Incandescent lamps, arc lamps, electron bombardment of phosphors (CRT or Braun tube), electroluminescence by impact ionization or tunnel injection, injection and subsequent light generating recombination in single crystal light emitting diodes (LEDs) or lasers (diode lasers) and plasma discharge tubes are a few examples of light sources which have, or potentially might be, used in displays.
Incandescent, arc and plasma techniques of light generation all require carefully controlled atmospheres within sealed envelopes and suffer from basic wear-out mechanisms. Electron bombardment of phosphors, or cathode luminescence, requires a low pressure (i.e. 10-6 Torr.) sealed envelope for electron acceleration and the prevention of arcing. In addition, cathode luminescence requires the emission of electrons into the low pressure region which may be accomplished with thermionic emission from heated filaments, field emission, or other high electric field techniques, or by the use of low work function materials such as the hydrogenated &lt;111&gt; face of diamond in conjunction with moderate electric fields.
A significant reduction in the required device drive voltage without the need for a vacuum envelope can be attained by building LEDs as the light emitting element of a display. Commercially available LEDs span the green--red portion of the spectrum with blue LEDs having been demonstrated. Since these LEDs are typically made of SiC, GaP, AlGaAs, InGaAlP, all single crystal materials with appropriate N/P and/or isoelectronic doping, large displays are typically made of arrays of discrete LEDs soldered into a large panel. Arraying discrete LEDs is costly and limits the pixel density. Large single crystals are expensive, available in limited sizes and contain defects that limit the potential yield of direct view display approaches based on using large single crystal substrates.
Some heterojunction approaches to light emitters have been discussed in the literature, such as the possibility of making LEDs by utilizing compounds of column II and VI of the periodic table as light emitting material and utilizing other II and VI compounds or group IV materials to form heterojunction LEDs. This literature cites dopability, band offset values and lattice match as the principal criterion for combining materials. Lattice match is especially stressed, with emphasis on the criterion that the materials must be approximately lattice-matched to allow the fabrication of high-quality, dislocation-free interfaces.